Some processes for the polymerization of propylene are carried out in the gas phase in fluidized or mechanically stirred bed reactors, in the presence of catalysts obtained from compounds of transition metals belonging to groups IV, V or VI of the Periodic Table of the Elements and aluminum alkyl compounds generating, in high yields, isotactic polypropylene being more than 95% wt insoluble in xylene at 25° C.
The polymer is obtained in the form of granules having a morphology depending on the morphology of the catalyst; the dimension of the granules, which depends on the original dimension of the catalyst particles and on reaction conditions, is distributed around an average value.
In these types of processes the heat of reaction is removed by a heat exchanger placed inside the reactor or in the recycle line of the reaction gas.
In some processes, a problem in polymerization processes of this type results from the presence of very fine polymer particles which are produced from pre-existing fine catalyst particles or breakage of the catalyst.
These fine particles tend to deposit onto, and electrostatically adhere to, the inner walls of the reactor and of the heat exchanger and then grow in size by polymerization causing an insulating effect and a lower heat transfer resulting in the formation of hot spots in the reactor.
These effects are enhanced when the gas-phase alpha-olefin polymerization process is carried out in the presence of highly active catalysts such as catalysts made from or containing the reaction product of an aluminum alkyl with a titanium compound supported on a magnesium halide in active form.
As a consequence, a loss in fluidization efficiency and homogeneity can occur. In some instances, catalyst feeding interruption may occur as well as clogging of the polymer discharge system. Furthermore, excessive temperature can result in particles melting with the formation of layers of thin agglomerates which adhere to the reactor walls and in the formation of agglomerates which may clog the gas distribution plate.
These drawbacks can lead to poor process reproducibility and a forced interruption to remove deposits which have formed inside the reactor even after relatively short times.
To reduce the extent of catalyst fragmentation, the catalyst can be subject to a pre-polymerization step carried out under mild conditions. In some instances, the pre-polymerization step is performed in a section of the plant immediately connected to the main polymerization section so that the prepolymer produced is directly fed to the main polymerization reactor (also called prepoly in-line) and is characterized by relatively high values of monomer conversion (50-2000 gpolymer/gcat). Alternatively, the pre-polymerization step is carried out in a dedicated section and the prepolymer produced is stored for future use. In this latter case, lower values of monomer conversion rates (0.1-50 g Polymer/gcat) are possible. In both cases and while the pre-polymerization may reduce the extent of improper catalyst fragmentation, the pre-polymerization does not reduce the negative effects of the polymerization activity derived from fine catalyst particles.